Inheritance of Pathogenicity in Melampsora lini

The flax-rust fungus, Melampsora lini (Pers.) Lév., is autoecious and long-cycle; that is, it produces its pycnial, aecial, uredial, and telial stages solely on species of flax. Allen (1) demonstrated the heterothallic nature of M. lini, and Flor (3, 4) found that rust of cultivated flax, Linum usitatissimum L., comprised numerous physiologic races. Every flax variety tested has been susceptible to 1 or more of 24 physiologic races identified from North and South American collections of flax rust. However, no single race known is capable of attacking all of the 11 differential varieties used in these tests. In the northern United States, flax rust overwinters by means of telia (6) and, consequently, natural hybridization is probably coincident with the initiation of infection each year. A sound program for the production of desirable rust-resistant varieties of flax requires knowledge not only of the interaction of the factors governing rust reaction possessed by the varieties of flax but also of the interaction of the factors for pathogenicity in various races of the flax-rust fungus. The data reported in the present paper deals with the latter subject. Knowledge of the inheritance of pathogenicity in the rusts is limited. It was not until 1927 that Craigie (2) demonstrated the heterothallic nature of rusts and thus made possible the genetic study of these organisms. Physiologic races of rust are differentiated by the type of infection produced on a series of differential varieties, a laborious process restricting the number of progeny cultures that can be studied. Additional impediments to the genetic study of pathogenicity in the rusts are the capricious germination of the telia and the difficulty of obtaining races of known homozygosity. It also is probable that the differential varieties used to identify the physiologic races are not capable of resolving the rust into all its genotypic strains. New physiologic races of cereal rusts have been obtained by selfing or crossing known races of Puccinia graminis tritici Eriks. and Henn. (8, 12, 13, 16), P. graminis avenae Eriks. and Henn. (10), and P. rubigo-vera tritici (Eriks. and Henn.) Carl. (17), as well as by crossing the different varieties of stem rust (14). The data of Johnson, Newton, and Brown (8), obtained in a study of the inheritance of pathogenicity in a cross between two races (9a x 36) of P. graminis tritici, point toward Mendelian inheritance. In this cross the pathogenicity of F₂ rust cultures to Arnautka Mindum, and Spelmar appeared to be inherited as a unit and was conditioned by a single pair of factors with virulence dominant. Pathogenicity to Kanred was conditioned by a single pair of factors with avirulence dominant, and pathogenicity to Vernal by 2 pairs of duplicate factors with avirulence dominant. Wach of these 3 characters appeared to be inherited independently. In a cross between races of P. graminis avenae, Johnson and Newton (10) found that in the F₁ avirulence was dominant to virulence.